The invention relates to an array and to a method for coating of objects, in particular plate-like objects such as semiconductor components, preferably Si wafers with surface coatings, comprising a device for supplying and transferring of at least one uncoated object to a coating device such as a chamber and for extracting and removing a coated object therefrom.
In the photovoltaic industry, systems for low-pressure gas phase deposition are used for, among other things, the manufacture of amorphous thin-film solar cells and for the manufacture of thin surface coatings on crystalline solar cells. PECVD, or plasma-enhanced chemical vapor deposition, is becoming increasingly preferred over thermal activation, since the latter is unsuitable for many applications because of the high temperatures needed (&gt;700.degree. C.) (see for example U.S. Pat. No. 5,626,677 or Vossen, Kern, Thin Film Processes, Academic Press, Inc., London, 1978, p. 337-342, or Rossnagel et al., Handbook of Plasma Processing Technology, Noyes Publications, New Jersey, 1990, p. 269-271, or Popov, High Density Plasma Sources, Noyes Publications, New Jersey, 1995, p. 408-410.
The following are known as industrially usable PECVD systems:
piping systems for simultaneous surface coating of more than 100 crystalline silicon wafers (Rossnagel et al., Handbook of Plasma Processing Technology, Noyes Publications, New Jersey, 1990, p. 269-271), PA1 closed continuous systems with air locks for coating large-area substrates (e.g. glass panes) or of large-area pallets that can be fitted with smaller silicon wafers (JP 0 731 6814 A), and PA1 open continuous systems without moving air locks for continuous coating of large-area substrates such as glass panes or steel strips (EP 0 574 178 A2). PA1 supply of an uncoated object to a first conveyor section using a transfer device at atmospheric pressure, PA1 transfer of the uncoated object to a second conveyor section, along which the atmospheric pressure is gradually reduced to a value that is adapted to the pressure necessary for coating of the object, PA1 transfer of the object to a third conveying device passing through a coating device, PA1 transfer of the coated object to a fourth conveyor section, inside which the pressure is gradually adjusted to the atmospheric pressure, PA1 transfer to the first conveyor section and removal of the coated object by the transfer device.
The aforementioned systems have however considerable drawbacks in respect of an inexpensive coating of two-dimensional objects, in particular crystalline silicon wafers.
In piping systems, a long boat of graphite plates is filled with up to 100 Si wafers and moved into a heated quartz glass tube. The individual graphite plates are electrically connected in pairs, so that when a voltage is applied a plasma burns between all the plates and leads to activation of the introduced process gases.
To achieve a high throughput, several alternatingly usable plasma tubes and a large number of "wafer boats" are required, involving heavy expenditure for the necessary conveying system for the boats.
The loading of fragile silicon wafers into the boats by robotic stations is a very cost-intensive process, since a large number of different positions must be loaded and the spacing between the individual plates in the boat is very narrow.
Not only the wafers, but also the required glass tube is coated with SiN. This entails an interruption after only a small number of coating operations in order to etch clean the walls of the glass tube. In addition, expensive and in some cases environmentally harmful etching gases (CFCs) are necessary.
Because of the high mass of the boats, long heating-up cycles are necessary that limit the throughput of the system.
In piping systems, deposition is only possible using parallel plasma plates. This necessitates a good electrical contact between the silicon wafers and the graphite plates. Catering for this requirement is posing more and more problems, since modern (inexpensive) silicon wafers for photovoltaic use are manufactured using methods that as a rule result in wavy substrates (band pulling method) (see for example Haefer, Oberflachen-und Dunnschicht-Technologie, Teil I Beschichtungen von Oberflachen, Springer-Verlag Berlin, 1987, p. 168-169).
The homogeneous coating of large wafers is problematic, since as the wafer size increases a homogeneous distribution of gas over the wafer and along the boat becomes more difficult.
An in-situ quality check is not possible. In the event of a system fault, the entire batch (of about 100 Si wafers) will be lost.
Closed continuous systems avoid many of the problems of piping systems; for example the use of modern plasma sources with a higher excitation density/deposition rate is possible, and an electrical contact to the wafer can be dispensed with. The problems of etching the facility clean is not problematic, since the source can, for example, be attached at the side of a vertically running pallet, so that falling particles cannot hit the wafer. Furthermore, a high surface coating quality can be obtained by the use of remote plasma sources. Despite these advantages, however, there are drawbacks that lead to high coating costs.
For example, the through-flow speed of the pallets is limited by the maximum coating rate of the plasma source used and by the maximum cycle rate of the air lock system used. An increase of these two quantities is only possible at considerable expense with the currently available technology. The pallets must therefore be designed very large in order to assure a required minimum throughput of the facility. This leads to a number of further problems, since a homogeneous coating of the pallet becomes more and more difficult as the size increases, the filling of a large pallet with individual wafers requires--as in piping systems--complicated and expensive robotics, and the entire facility assumes very large proportions. The latter problem inevitably leads to high investment costs.
Open continuous systems without moving air locks for the coating of crystalline silicon wafers are not currently used industrially.
Here the solar cells must lie in carriers that pass in a gap-free and endless line in self-sealing manner through a channel with gradually decreasing pressure. Very high demands must be placed on the solar cell carriers as regards their mechanical stability, so that they can exercise a sufficient sealing function. This is problematic in respect of the following two aspects and therefore leads to high manufacturing costs.
The carriers are subjected to extreme temperature changes, since the coating of the solar cells as a rule takes place at temperatures above 300.degree. C. There is a risk that they become warped as a result.
The carriers are also coated. This changes their sliding properties and their geometrical dimensions. In addition, the carriers must be freed of this surface coating in the course of maintenance work. For carriers, which are precision tools, this process must be performed very carefully, and is consequently cost-intensive. If for design reasons (friction, thermal expansion etc.) high tolerances in the gap width between carrier and guide channel are unavoidable, a relatively strong air current results. This current can be completely drawn off using large vacuum pumps, but there are the risks that the wafers might start to wobble under the current and break, or that an excessive number of particles might collect in the facility.